Raman spectroscopy is a powerful and effective tool for analytical studies of biological and chemical samples. Raman scattering is inelastic light scattering from a sample that may yield a molecular fingerprint of the constituent molecules. An inherent limitation of this technique is the low Raman cross section of bio-molecules. Hence, long integration times are required to obtain good signal to noise ratio. Nevertheless, Raman spectra have rich information content and a single Raman spectrum can provide information about all the molecular constituents of the sample.
Raman spectroscopy has been combined with microfluidic system. To overcome the limitation of the inherently low Raman cross section, Surface Enhanced Raman Spectroscopy (SERS) based detection schemes have been employed in microfluidic systems. Other experiments have used confocal Raman microscopy for online monitoring of chemical reactions. In all these applications, monitoring was performed through a combination of a bulk Raman microscope and a microfluidic chip. A problem of using microscope based systems to collect Raman data from such microfluidic chips is that the signal is acquired through a substrate which has its own background signal. This limits the detection efficiency of the system. Using a microscope also precludes miniaturization.
FIG. 1 shows an example of a known Raman fiber probe. This has a central excitation fiber and several collection fibers arranged around it. All fibers carry an appropriate filter at their tips for Raman spectroscopy applications: a bandpass filter for the excitation fiber, and a longpass filter for the collection fibers. In this arrangement, the head of the probe is shared by both the excitation and the collection fibers, with both fibers facing in the same direction. An advantage of this design is its overall small shape. A disadvantage is that it is relatively complex and expensive to make. In addition, the Raman excitation and collection fibers are bundled together and lack flexibility to inspect samples at different angles between the collection probe and the excitation probe. A further disadvantage is that the configuration is not optimized for the filters. The filter works at its maximum efficiency when the beam is perpendicular to the entrance surface of the filter. Hence, in this design, where a non-collimated beam passes through the filters, filter efficiency is reduced.
FIG. 2 shows an example of another probe. In this case, there is a single excitation fiber and a single collection fiber arranged in separate parallel tubes. At the output of the excitation fiber is a bandpass filter, a dichroic mirror, and lens, which focuses the excitation light onto a sample area. Light incident on the sample causes Raman excitation and consequently the emission of a Raman signal. The Raman signal passes through a long pass filter for collection in the collection fiber. As with the arrangement of FIG. 1, in the device of FIG. 2, the head of the probe is shared by both the excitation and the collection fibers, with both fibers facing in the same direction. An advantage of this design is its overall small shape. However, whilst this arrangement is more appropriate for the filters because of the collimated beam, it is difficult to manufacture and provides limited flexibility in terms of angles of excitation and collection.